Multilayer adhesive for thermal reversible joining of substrates

ABSTRACT

One embodiment of the invention includes a multilayer dry adhesive system capable of reversible joining of rigid substrates.

This application claims the benefit of U.S. Provisional Application No.60/939,680, filed May 23, 2007.

TECHNICAL FIELD

The field to which the disclosure generally relates includes multilayeradhesives, products including the same, and methods of making and usingmultilayer adhesives.

BACKGROUND

Gecko feet pads, with nanohair structures on them, are examples of smartdry adhesives. The working principle of the Gecko adhesion is that thenanohair structure allows the foot pad to make maximum contact with acounter surface regardless of its roughness and chemical composition.This is accomplished by nanohairs that are relatively long andprotruding from the foot pad at an angle so that adjacent nanohairs cancontact the counter surface regardless of its topography. The maximumcontact further allows for accumulation of millions of small van derWaals (in the range of microNewtons) interactions between the Gecko footpad and the counter surface, leading to an overall adhesion force(pull-off force) of about 10 N/cm². When the detaching force is employedin a peel-off mode, however, the complete detachment is achievedgradually by overcoming small adhesion forces corresponding to verysmall areas. Thus, the adhesion is easily reversed. Overall, theattractiveness of the Gecko adhesion lies in the combination of adhesivestrength (10 N/cm²), reversibility, and the ability to adapt to avariety of surfaces in terms of both the surface roughness andcomposition. The above unique features of the Gecko adhesion hasstimulated scientific research efforts to produce synthetic smart dryadhesives that work using the same principle as the Gecko feet. Up tonow, the two best synthetic Gecko adhesives show maximum pull-offstrength of 3 and 10 N/cm² towards glass. Both adhesives suffer fromsevere adhesion loss after only one or two attaching/detaching cycles,as a result of breakdown or lateral collapse of the nano structures,with the latter referring to the bonding of adjacent nano-hairs. Inaddition, typical synthetic Gecko adhesives only mimic the function ofthe gecko footpad and function more like adhesive tapes. The mechanicaldetachment of the footpad by gecko toes is a feature not found intypical synthetic gecko adhesives. If a double side tape-like syntheticgecko adhesive is used to bond two rigid substrates, the bonding is notreversible due to the constrain of the rigid backing layers on bothsides.

SUMMARY OF EXEMPLARY EMBODIMENTS OF THE INVENTION

One embodiment of the invention includes a thermo-reversible multilayerdry adhesive system capable of reversible joining of rigid substrates.

Other exemplary embodiments of the invention will become apparent fromthe detailed description provided hereinafter. It should be understoodthat the detailed description and specific examples, while disclosingexemplary embodiments of the invention, are intended for purposes ofillustration only and are not intended to limit the scope of theinvention.

BRIEF DESCRIPTION OF THE DRAWINGS

Exemplary embodiments of the present invention will become more fullyunderstood from the detailed description and the accompanying drawings,wherein:

FIG. 1 shows a method of attaching a first substrate to a secondsubstrate utilizing a multilayer adhesive, according to one embodimentof the invention.

FIG. 2 shows a method of attaching a first substrate to a secondsubstrate utilizing a multilayer adhesive, according to one embodimentof the invention.

FIG. 3 shows a method of attaching a first substrate to a secondsubstrate utilizing a multilayer adhesive, according to one embodimentof the invention.

DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS

The following description of the embodiment(s) is merely exemplary innature and is in no way intended to limit the invention, itsapplication, or uses.

Referring now to FIGS. 1-2, one embodiment of the invention includes aproduct including a thermo-reversible multilayer adhesive 14 having ashape memory polymer (SMP) layer 18, a first dry adhesive layer 16, anda second dry adhesive layer 20. The first dry adhesive layer 16 mayoverlie the shape memory polymer layer 18, and the second dry adhesive20 may underlie the shape memory polymer layer 18. In one embodiment ofthe invention, the first dry adhesive layer 16 and second dry adhesivelayer 20 have different profiles. In one embodiment, the first dryadhesive layer 16 may be thinner than the second dry adhesive layer 20.In one embodiment, the multilayer adhesive 14 may be a triple layeradhesive.

As shown in FIG. 1, one embodiment of the invention may include a methodof attaching a first substrate 10 to a second substrate 12 utilizing themultilayer adhesive 14. In one embodiment, the first dry adhesive layer16 has a thickness less than the second dry adhesive layer 20.

In one embodiment, the multilayer adhesive 14 is positioned between thefirst substrate 10 and the second substrate 12. Then the multilayeradhesive 14 is heated above the glass transition temperature (T_(g)) ofthe shape memory polymer layer 18 and cooled under a load so that thefirst substrate 10 is attached to the second substrate 12. Thereafter,the first substrate 10 and second substrate 12 may be detached from eachother by heating the multilayer adhesive 14 above the glass transitiontemperature (T_(g)) of the shape memory polymer layer 18. Because thethicknesses of the dry adhesive layers 16 and 20 may be different, theasymmetrical multilayer adhesive may have a curvature with the curveddry adhesive surface pointing in the same direction as shown in FIG. 1.For example, the outer surface of each of the first dry adhesive layer16 and the second dry adhesive layer 20 has a generally concave shape.Upon heating and cooling under a load, the multilayer adhesive deformsand a maximum pull-off strength can be achieved. Upon heating the shapememory polymer layer 18 above the T_(g) thereof, the shape memorypolymer layer 18 has a tendency to return to its original curvature,which in principle would create a peeling force to detach the second dryadhesive layer 20 from the second substrate 12. In reality, this peelingof dry adhesive 20 from the substrate 12 would require the spontaneousdetachment of the first dry adhesive 16 from the substrate 10, which isnot permitted by the peeling detachment mode given the particulargeometry in FIG. 1. This means that the detachment of the dry adhesive16 from substrate 10 will have to work against the strong pull-offstrength between the substrate 10 and the first dry adhesive layer 16.Such a separation mechanism is not favorable and only possible when theshape recovery force of the shape memory polymer layer 18 is strongerthan the pull-off-strength of the first dry adhesive layer 16.

Referring now to FIG. 2, another embodiment of the invention includesthe multilayer adhesive 14 with a geometry wherein the curvature of thefirst and second adhesive layers 16, 20 point in the oppositedirections. For example the outer surface of each of the first dryadhesive layer 16 and the second dry adhesive layer 20 has a generallyconvex shape. As such, the shape memory polymer layer 18 has aconfiguration wherein the middle portion thereof is the thickest andnarrows or is tapered toward the outer edges of the shape memory polymerlayer 18. The multilayer adhesive 14 is placed between the firstsubstrate 10 and the second substrate 12 and heated and then cooledunder a load to attach the first substrate 10 to the second substrate12. The first substrate 10 and the second substrate 12 may be detachedfrom each other by heating the multilayer adhesive 14. Upon heating,this particular geometric design of the multilayer adhesive 14, inprinciple, would allow the thermo-activated peeling detachment mode onboth adhesive sides, which is not possible with the design in FIG. 1.Each of the first and second dry adhesive layers 16 and 20 peel from thesubstrates 10, 12 respectively, first from near outer edges 22 and 24,and progressing towards a center 26. Although the design in FIG. 2 doespermit peeling on both sides for adhesive detachment, the adhesivebonding would require a very large load to deform the multilayeradhesive 14, as the deformation of shape memory polymer layer 18 duringbonding is compressive.

In one embodiment, the first substrate 10 and the second substrate 12may be flat and the multilayer adhesive 14 may be curved. In anotherembodiment, the first substrate 10 and the second substrate 12 may becurved and the multilayer adhesive 14 may be flat.

Referring now to FIG. 3, another embodiment of the invention includesthe multilayer adhesive 14 having the shape memory polymer layer 18, thefirst dry adhesive layer 16, and the second dry adhesive layer 20. Themultilayer adhesive has a geometry wherein the curvature of the firstand second adhesive layers 16, 20 point in the opposite directions. Forexample the outer surface of each of the first dry adhesive layer 16 andthe second dry adhesive layer 20 has a generally convex shape. The firstand second adhesive layers 16, 20 meet each other at the outer edges 22and 24. The shape memory polymer layer 18 comprises a first portion 15adjacent to the first adhesive layer 16 and a second portion 17 adjacentto the second adhesive layer 20, and a cavity 19 is formed between thefirst portion 15 and the second portion 17. In one embodiment, the shapememory polymer layer 18 has a geometry thickest in the center portion ofthe shape memory polymer and tapering to the outer edges thereof. Thethickness of the cavity 19 may also be the greatest in the centerportion and tapering to the outer edges thereof.

In one embodiment, the multilayer adhesive 14 is placed between thefirst substrate 10 and the second substrate 12 and heated above theT_(g) of the shape memory polymer 18 and then cooled under a load toattach the first substrate 10 to the second substrate 12. Upon coolingunder the load, the cavity 19 may become very small or may disappearaltogether. The first substrate 10 and the second substrate 12 may bedetached from each other by heating the multilayer adhesive 14 above theT_(g) of the shape memory polymer 18. Upon heating, each of the firstand second dry adhesive layers 16 and 20 peel from the substrates 10, 12respectively, first from near outer edges 22 and 24, and progressingtowards a center 26. Upon heating, the cavity 19 forms in between thefirst portion 15 and the second portion 17 of the shape memory polymerlayer 18. This design shown in FIG. 3 allows the peeling on both sidesof the multilayer adhesive 14 and the bonding does not require a verylarge load as the deformation of the shape memory polymer 18 duringbonding is bending in nature.

Another embodiment of the invention includes a method comprising heatingthe multilayer thermo-reversible dry adhesive 14 including the shapememory polymer layer 18, the first dry adhesive layer 16, and the seconddry adhesive layer 20, and wherein the multilayer adhesive 14 has acurved structure at room temperature and applying a load to the adhesiveso that the adhesive adheres to an underlying substrate so that theadhered adhesive has a pull-off force greater than 10 N/cm², andthereafter detaching the adhesive comprising heating the adhesive to atemperature above the T_(g) of the shape memory polymer 18 to cause theadhesive to return to a curved structure.

The dry adhesive layers 16 and 20 may provide a continuous contactsurface or the dry adhesive layers may include a plurality of spacedapart fingers each providing a relative small contact surface so theoverall contact surface of the adhesive layer is not continuous.

One embodiment includes a multilayer dry adhesive system capable ofreversible joining of rigid substrates having a pull-off adhesion forcefor joining substantially higher than a peeling-off force for dejoiningor unjoining or decoupling.

Numerous shape memory polymers may be utilized in various embodiments ofthe invention. For example, starting with a typical aromaticdiepoxy/diamine system with a T_(g) of about 90° C., the aromatic epoxycomponent is replaced systematically with an aliphatic diepoxy to yielda series of epoxy shape memory polymers with T_(g)'s ranging from 3° C.to 90° C. As such, a shape memory polymer may be tailored for use with adry adhesive as desired for a particular application operated withincertain temperature ranges. As such, the dry adhesive layer may have aT_(g) ranging from −90° C. to 200° C., and the shape memory polymer mayhave a T_(g) ranging from 25° C. to 200° C.

Some embodiments refer to a multilayer epoxy dry adhesive. It should beunderstood that more than two layers may be utilized. For example, theremay be two or more layers of the shape memory polymer layer, which maybe in a side by side relationship or an overlying relationship.Likewise, there may be two or more dry adhesive layers in a side by siderelationship or an overlying relationship. As such, a device with customor tailored properties may be manufactured.

In various embodiments, the dry adhesive layers may be an epoxyelastomeric dry adhesive. In various embodiments, the shape memorypolymer may be an epoxy. In various embodiments of the invention, anycombination of a shape memory polymer and a dry adhesive may beutilized. The following examples of shape memory polymers and dryadhesives are for illustrative purposes only.

In various embodiments, the components of the dry adhesive or thecomponents of the shape memory polymer may include a rigid epoxy and aflexible epoxy. The range of possible crosslinking chemistries which maybe used to achieve a dry adhesive or shape memory polymer may includealpha, omega-diaminoalkanes, organic multi-carboxylic acid, anhydride,or catalytic (as in imidazole type) crosslinking reactions. There aremany different ways to achieve the appropriate relationships between themolecular properties. For example, the dry adhesives or shape memorypolymers may include a rigid epoxy, an epoxy extender, and acrosslinking agent; or a rigid epoxy, a flexible crosslinking agent, anda flexible epoxy; or a rigid epoxy, a rigid crosslinking agent, and aflexible epoxy; or a rigid epoxy, a flexible epoxy, and a catalyticcuring agent; or a rigid epoxy, a crosslinking agent, and a diluent; ora flexible epoxy, a crosslinking agent, and a diluent; or a rigid epoxyand a flexible crosslinking agent; or a flexible epoxy and a catalyticcuring agent; or a flexible epoxy and a crosslinking agent; and whereinthe rigid epoxy is an aromatic epoxy having at least two epoxide groups,the flexible epoxy is an aliphatic epoxy having at least two epoxidegroups, the epoxy extender has one epoxide group, and the crosslinkingagent is one of a multi-amine, an organic multi-carboxylic acid, or ananhydride, and the diluent is a monoamine or a mono-carboxylic acid. Invarious embodiments, the catalytic curing agent (or catalytic cure)promotes epoxy-to-epoxy or epoxy-to-hydroxyl reactions. The catalyticcuring agent may include, but is not limited to, tertiary amines, aminesalts, boron trifluoride complexes, or amine borates. In one embodiment,the components of the dry adhesive may be present in an amountsufficient to provide, upon curing of the composition, a dry adhesivehaving a glass transition temperature of −90° C. to 200° C. and having apull-off strength of 1-200 N/cm² from a substrate. In one embodiment,the components of the shape memory polymer composition may be present inan amount sufficient to provide, upon curing of the composition, anepoxy shape memory polymer having a change in storage modulus of 2 to 3orders of magnitude before and after its glass transition.

One embodiment of the invention includes a method of making a multilayerthermo-reversible dry adhesive comprising heating 3.6 g of EPON 826 (aBisphenol A based epoxy resin) to about 75° C. and mixing the same with2.16 g of neopentyl glycol diglycidyl ether (NGDE) and 2.3 g of adiamine such as Jeffamine D-230. Jeffamine D-230 is a polyetheraminethat is difunctional, primary amine with an average molecular weight ofabout 230. The primary amine groups are located on secondary carbons atthe end of the aliphatic polyether chain. Jeffamine is available fromHuntsman.

The mixture may then be cured in a specially designed mold to create anepoxy shape memory polymer layer having geometry shown in FIG. 3. Thecuring condition is 100° C. for 1.5 hours. Then a mixture of 2.16 g ofNGDE and 1.15 g of an amine such as Jeffamine D-230 is cured on theouter surface of the shape memory polymer layer for 1.5 hours at 100° C.In a third step, the oven temperature may be raised to 130° C. forpost-curing for about one hour. At the end of the post-curing, the curedmultilayer epoxy is demolded. Overall, the favored curved structureshown in FIG. 3 is created by molding As will become evident hereafter,the curved structure is advantageous in providing a reversibility of theadhesion.

Numerous shaped memory polymers may be utilized in various embodimentsof the invention. For example, staring with a typical aromaticdiepoxy/diamine system with a T_(g) of about 90° C., the aromatic epoxycomponent is replaced systematically with an aliphatic diepoxy to yielda series of epoxy shape memory polymers with T_(g)'s ranging from 3° C.to 90° C. As such, a shape memory polymer may be tailored for use with adry adhesive as desired for a particular application operated withincertain temperature ranges.

The following is another embodiment of the invention providing a methodof making a shape memory polymer. EPON 826 was weighed into a glassbottle and placed into an oven preset at 70° C. to melt. The meltingtook about 1 hour. Immediately after the bottle containing the EPON 826was taken out of the oven, weighed Jeffamine D-230 and NGDE were addedto the bottle. The bottle was then shaken vigorously by hand for aboutten seconds to mix the components. The detailed formulations of the fiveepoxy SMP samples prepared according to the method are summarized inTable 1.

TABLE 1 Formulations of epoxy samples 1-5 EPON 826 NGDE Jeffamine D-230Sample # (mole) (mole) (mole) 1 0 0.02 0.01 2 0.005 0.015 0.01 3 0.010.01 0.01 4 0.015 0.005 0.01 5 0.02 0 0.01

Next, the mixture was poured into a mold. The epoxy samples werethermally cured at 100° C. for 1.5 hours and postcured at 130° C. for 1hour. Upon the completion of the cure, the epoxy samples were demolded.The geometry of the shape memory polymer layer obtained is determined bythe mold used.

In another embodiment, the system consists of EPON 826, Jeffamine D-230as the crosslinker, and decylamine as the monoamine. As shown in Table2, from sample 6 to 11, the fraction of the crosslinker issystematically reduced, while the total amounts of epoxy functionalityand active hydrogen functionality on the amines are maintained equal.Among these samples, sample 11 was used as a reference sample because itcontains no crosslinker and is not expected to possess shape memoryproperties. The shape memory epoxy polymers according to theformulations in Table 2 have T_(g)'s from 25° C. to 90° C.

TABLE 2 Formulations of epoxy samples 6-11 EPON 826 Jeffamine D-230decylamine Sample # (mole) (mole) (mole) 6 0.02 0.01 0 7 0.02 0.00750.005 8 0.02 0.005 0.01 9 0.02 0.0025 0.015 10 0.02 0.0005 0.019 11 0.020 0.02

The above description of embodiments of the invention is merelyexemplary in nature and, thus, variations thereof are not to be regardedas a departure from the spirit and scope of the invention.

1. A method comprising: placing a multiplayer adhesive system between a first substrate and a second substrate, the multiplayer adhesive system comprising a first dry adhesive layer having a first curved profile, a second dry adhesive layer having a second curved profile, and shape memory polymer layer therebetween; heating the multiplayer adhesive system; pressing the substrates together with the heated multiplayer adhesive system therebetween and cooling the multiplayer adhesive system under load to attach the first substrate to the second substrate; and thereafter heating the multiplayer adhesive system so that the multiplayer adhesive system changes shape and peels away from the substrates from the outer edges of the multilayer dry adhesive system.
 2. A method as set forth in claim 1 wherein each of the first dry adhesive layer and the second dry adhesive layer has a generally convex curved profile.
 3. A method as set forth in claim 1 wherein the shape memory polymer layer comprises: at least one of a rigid epoxy or a flexible epoxy; and at least one of a crosslinking agent or a catalytic curing agent; wherein the rigid epoxy is an aromatic epoxy having at least two epoxide groups, the flexible epoxy is an aliphatic epoxy having at least two epoxide groups, and the crosslinking agent is one of a multi-amine, an organic multi-carboxylic acid, or an anhydride.
 4. A method as set forth in claim 1 wherein the first adhesive layer and the second adhesive layer comprises: at least one of a rigid epoxy or a flexible epoxy; and at least one of a crosslinking agent or a catalytic curing agent; wherein the rigid epoxy is an aromatic epoxy having at least two epoxide groups, the flexible epoxy is an aliphatic epoxy having at least two epoxide groups, and the crosslinking agent is one of a multi-amine, an organic multi-carboxylic acid, or an anhydride.
 5. A method comprising: placing a multilayer adhesive system between a first substrate and a second substrate, the multilayer adhesive system comprising a first dry adhesive layer, a second dry adhesive layer, and shape memory polymer layer therebetween, wherein the shape memory polymer layer comprises a first portion adjacent to the first dry adhesive layer and a second portion adjacent to the second dry adhesive layer and a cavity therebetween, wherein an outer surface of each of the first dry adhesive layer and the second dry adhesive layer has a convex shape, and wherein the thickness of the cavity is the greatest in a center portion of the multilayer adhesive system and tapers to outer edges thereof; heating the multilayer adhesive system; pressing the substrates together with the heated multilayer adhesive system therebetween and cooling the multilayer adhesive system under load to attach the first substrate to the second substrate; and thereafter heating the multilayer adhesive system so that the multilayer adhesive system changes shape and peels away from the substrates from the outer edges of the multilayer dry adhesive system.
 6. A method comprising: placing a multilayer adhesive system between a first substrate and a second substrate, the multilayer adhesive system comprising a first dry adhesive layer, a second dry adhesive layer, and shape memory polymer layer therebetween, wherein the shape memory polymer layer has a geometry thickest in a center portion of the shape memory polymer layer and tapering to the outer edges thereof; heating the multilayer adhesive system; pressing the substrates together with the heated multilayer adhesive system therebetween and cooling the multilayer adhesive system under load to attach the first substrate to the second substrate; and thereafter heating the multilayer adhesive system so that the multilayer adhesive system changes shape and peels away from the substrates from the outer edges of the multilayer dry adhesive system.
 7. A method as set forth in claim 1, wherein said shape memory polymer has a glass transition temperature and said heating the multiplayer adhesive system comprises heating to a temperature above the glass transition temperature.
 8. A method as set forth in claim 1, wherein said multiplayer adhesive system comprises at least two shape memory polymer layers. 